The attractiveness of ceramics as materials for components of high temperature engines is widely recognized. The heat exchanger is an important part of gas turbine engine designs, since it recovers waste heat losses and preheats incoming air in order to improve engine efficiency. Both fuel consumption and exhaust emissions are minimized. Heat exchangers for furnace waste heat recovery are also receiving increased attention. Ceramic heat exchangers have the advantage of high temperature capabilty and corrosion resistance and are lighter in weight and potentially lower in cost than superalloy materials.
Ceramic materials which have been considered for use as heat exchanger materials are lithia-alumina-silica (LAS) and magnesia-alumina-silica (MAS) ceramics because of their low thermal expansion coefficients and correspondingly high thermal shock resistance. At present, MAS materials appear to exhibit better chemical durability than LAS materials under engine operating environments. Recent efforts have been directed to the lowering of the thermal expansion coefficients of MAS bodies in order to optimize thermal shock resistance. In U.S. Pat. No. 3,979,216, issued Sept. 7, 1976, and assigned to the present assignee, there is described a cordierite body having a microstructure characterized by the presence of intentionally introduced microcracks. In U.S. Pat. No. 4,063,955, issued Dec. 20, 1977, and assigned to the present assignee, a low expansion cordierite body is produced from a glass frit. While both of these approaches have been successful in producing low expansion cordierite bodies, a method has been sought which does not depend upon the introducton of microcracks, which could lead to lower than desired mechanical strength, or upon the presence of glassy material, incomplete crystallization of which could increase thermal expansion to undersirably high levels.